This invention relates to the elimination of echos in communication signal paths and, more particularly, to effective cancellation of echos in communication paths which subject the echo signal to time-varying transmission impairments.
In communication paths via carrier systems, a signal returned because of an impedance mismatch at a hybrid junction is subjectively termed an echo due to its delayed arrival at the signal source. The returned signal may experience time-varying transmission impairments such as frequency offset and phase jitter. Frequency offset in an echo path can be produced in certain types of frequency division multiplexed transmission systems wherein a common carrier supply is not used for both transmitter and receiver terminals at one end of a transmission system and that system is located between the echo control device and the hybrid junction. Phase jitter refers to spurious variations in phase between successive portions of a repetitive signal in relation to the phase of a continuous oscillation at a fixed frequency and may also result from a variety of effects in different carrier systems which make up a portion of the echo path. The typical carrier system includes two one-way signal paths wherein phase differences may also arise between the carrier signals of the two paths because of time-varying signal propagation delays. Accordingly, a versatile echo canceller must compensate for time-varying phase impairments in the channel to provide adequate echo cancellation.
One approach to this problem is to increase the speed of the adaptive process performed by the echo canceller to accommodate time variations in the impulse response of the echo path in addition to the always-present, somewhat time invariant, characteristics of the impulse response. This may be done by increasing the incremental size, or gain, of the adaptation process; however, this approach is inconsistent with minimization of residual echo since speed of operation necessary to accommodate time variations is traded for precise operation. Precise operation is essential to refine the adaptation process to provide an accurate replica of the echo signal and minimize residual echo. Rapid convergence also tends to decrease the stability of echo cancellers and increases undesirable adaptation such as to noise and double-talking, i.e., intervals in which signals are transmitted by both one-way signal paths at the same time.